1. Field of the Invention
This invention relates to supported vanadium phosphorus oxide catalysts and a process for their preparation.
2. Technical Background
Maleic anhydride is used as a raw material for numerous products, including agricultural chemicals, paints, paper sizing, food additives and synthetic resins. To fill the high demand for this valuable chemical, a variety of commercial processes have been developed for its production, the most successful of which involves the vapor phase oxidation of n-butane to maleic anhydride in the presence of a vanadium phosphorus oxide (xe2x80x9cVPOxe2x80x9d) catalyst. Since the development of this method in the 1970""s, research has continued to improve the reaction conditions and, particularly, the VPO catalysts.
A review of the improvements made in this technology is given by G. J. Hutchings, Applied Catalysis, 72(1991), Elsevier Science Publishers B. V. Amsterdam, pages 1-31. The preferred method of preparation of VPO catalysts is the hydrochloric acid digestion of V2O5 and H3PO4 in either an aqueous solvent, as described, for example, in U.S. Pat. No. 3,985,775, or non aqueous solvent, such as methanol, tetrahydrofuran (THF) or isobutanol, followed by solvent removal to give what is termed the catalyst precursor, vanadium hydrogen phosphate, VO(HOPO4).(H2O)0.5. The precursor is then activated by heating, as described, for example, in U.S. Pat. No. 3,864,280 and U.S. Pat. No. 4,043,943. Further optimization of the preparation is described in U.S. Pat. No. 4,132,670, whereby vanadium pentoxide is heated with a selected anhydrous unsubstituted alcohol, adding an orthophosphoric acid to form the catalyst precursor and calcining the precursor to obtain the catalyst having high intrinsic surface area. Further attempts to improve the VPO catalyst performance by the use of dopants and/or supports are described in U.S. Pat. No. 4,442,226 and U.S. Pat. No. 4,778,890.
Vanadium, phosphorus and oxygen can form a large number of distinct compounds which have been well characterized, e.g., xcex1-VOPO4, xcex3-VOPO4, VOHPO4, (VO)2P2O7, VO(PO3)2 and VO(H2PO4)2. The most active catalytic phase is believed to be (VO)2P2O7, which is also the predominant oxide phase in VPO catalysts. Nevertheless, VPO catalysts are usually referred to as xe2x80x9cmixed oxidesxe2x80x9d in recognition of the probable presence of other oxide phases. VPO catalysts typically have V:P atomic ratios in the range of 1:1 to 1:2 and have an average bulk vanadium oxidation state in the range of 4.0-4.3.
Guliants et al., Catalysis Today, 28 (1996), pages 275-295, studied the effect of the phase composition of VPO catalysts on their effectiveness as catalysts for the oxidation of n-butane to maleic anhydride. This work indicated that the best performihng VPO catalyst was prepared from vanadyl hydrogen phosphate hemihydrate precursor that was free of microcrystalline or amorphous phases, such as VO(H2PO4)2 and xcex4-and xcex3-vanadyl (V) orthophosphates. It was disclosed that these undesirable components could be removed by washing either the precursor or the catalyst with boiling water.
While numerous modifications have been made to improve performance of VPO catalysts, VPO has a low thermal conductivity. With the high temperatures of reaction used in the vapor phase oxidation of n-butane to maleic anhydride and the large amounts of heat released, the catalysts deteriorate in activity over time.
In one aspect, the invention provides a catalyst comprising vanadium phosphorus oxide combined with a thermally conductive material. Vanadium phosphorus oxide compounds can be exemplified by vanadyl pyrophosphate, however, it should be noted that any vanadium phosphorus oxide compounds which are catalytically active can be used in the catalyst. The thermally conductive material has a thermal conductivity of at least 1 W meterxe2x88x921 Kxe2x88x921. Typically the conductive material is selected from the group consisting of silicon nitride, boron nitride, phosphorus treated boron nitride, aluminum nitride and mixtures thereof.
In another aspect, the invention comprises a process for preparing a catalyst comprising vanadium phosphorus oxide combined with a thermally conductive material, the process comprising the steps of:
a) forming a suspension comprising a vanadium (IV) phosphate compound in a liquid medium;
b) adding a thermally conductive material to the suspension under agitation at a temperature between 40xc2x0 C. and 120xc2x0 C. to provide vanadium phosphorus oxide precursor combined with the thermally conductive material;
c) drying the vanadium hydrogen phosphate precursor /thermally conductive material;
d) optionally but preferably washing the dried vanadium phosphorus oxide precursor/thermally conductive material with water;
e) calcining the vanadium phosphorus oxide precursor at elevated temperature (150xc2x0 C., for 12-15 hours) to obtain a catalyst comprising vanadium phosphorus oxide combined with a thermally conductive material;
f) calcination at 380xc2x0 C., hours, air;
g) activation in butane/air (1.5% butane/air =13.1% O2/butane), for 15 hours;
h) additional activation at 420xc2x0 C., 1020 h-1 (1.5% butane/air) for 100 hours; and
i) stabilization for an additional 50 hours under reaction conditions.
It is an object of this invention is to further advance the technology of VPO catalysis by providing for a VPO catalyst combined with a thermally conductive material particularly effective for hydrocarbon oxidation.